1. Field of the Invention
This invention relates to offset correction for use in conjunction with detectors and primarily infrared detectors.
2. Brief Description of the Prior Art
Detector systems generally include a plurality of detector elements, generally arranged in a column or a matrix, which scan or stare at a scene and generate a signal from each detector element (generally referred to as a "pixel") indicative of the portion of the scene detected by that detector element at a particular point in time. The full scene is a composite of the signals received from all or a portion of the detectors elements. In order for the full scene to be depicted accurately, it is necessary that each of the detector elements provide an identical output for the same input. This is generally not the case because most detectors have a different response to the same input thereto. This requires that each detector element be calibrated and that its output be adjusted to normalize the system, hence the use of offset correction to calibrate each of the detector elements. This calibration takes place while the detector array is operating in a system and is a part of the circuitry of the system.
Offset correction takes many forms, one such form involving the use of a thermal source and a chopper whereby the thermal source is used as the common scene for all of the detector elements for calibration. Since all of the detector elements are provided with an identical scene, any differences from detector element to detector element can be measured and an offset provided to each detector element to compensate for the differences and normalize the output of the system. A problem with this technique is that the detector system cannot view the scene continually because recalibration is required periodically, at which time the thermal source is substituted for the scene. Furthermore, the use of the thermal source and chopper adds substantial bulk and cost to the system. A yet further problem is that the outputs of one or more of the detector elements can change after initial calibration for various reasons, thereby making the existing factory installed offset inaccurate. In addition, calibration takes place at an artificial flux level rather than at the true scene flux level, so calibration is not necessarily taking place under the same conditions as the actual operation. It is therefore apparent that other types of detector calibration would be desirable.
In this regard, dither offset has been developed. Prior art systems of this type are set forth in a paper entitled "Dithered Scan Detector Compensation" by William F. O'Neil, Proc. IRIS Passive Sensors, 1992, Vol. 1, pp. 123-134, the contents of which are incorporated herein by reference. This reference sets forth systems for sensing images using a detector array and correcting errors in the image signals caused by gain and offset variations from detector to detector in the array. To correct gain and offset errors, the detector array is dithered by moving the detector line of sight between consecutive frames according to a predetermined pattern. This dithering causes different detectors to image the same location in the scene during different frames and causes two adjacent detectors to scan between the same two points in the scene during a cycle of the dither pattern. Image data generated from the dithering is used to remove gain and offset errors from the sensed images and to generate gain and offset correction values to be stored in a table and applied to the sensed images. The system continually recalibrates and maintains the correction immune to scene changes when the detector array is installed on a moving platform.
The above described approach of O'Neil updates the offsets every four frames using information only from four neighboring detectors to update the offset correction for a given pixel. This approach provides relatively slow correction for two reasons, these being (1) correction is only updated every four frames and, more important, (2) only a small amount of the available information is used in computing the correction.